I will give an argument that many people in our society are operating with a concept of marriage that differs in a central way from the traditional Western concept of marriage. I will then say a little on the same-sex marriage debate.
Entry into an institution is largely or wholly defined normatively by what participation in the institution makes permissible, obligatory or impermissible. Some of these normative features of institutions are central to the role that the institution plays and some may not be so central.
In the Western tradition, one of the normative features of the institution of marriage is that it bestows on the couple the permission to engage in sexual relations. Moreover, this normative feature has been quite central to the institution of marriage in the West. For instance, this permission to have sex has very often been one of the main reasons for a couple to marry (St. Paul certainly sees it this way). In fact, of the permissions that the institution of marriage bestows on a couple, it is hard to find any others that are of equal centrality. Marriage bestows a permission to call oneself "married", but that is mainly a word. Most of the other permissions, such as those involving taxation, visitation, living under one roof, etc. are clearly much more contingent in the Western tradition.
But presently, many believe that it can be permissible for unmarried people to have sex. Therefore, entry into marriage is presumably not seen by them as the bestowal of the permission to engage in sexual relations. It is hard to deny that this is a change in a central feature of the concept of marriage. In the past, marriage's normative impact in the sexual sphere of activity was that something previously impermissible—sexual relations—became permissible. Thus, marriage significantly expanded the options for sexual activity. In the present day, marriage's normative impact in the sexual sphere is seen by many as a contraction—rather than allowing previously forbidden sexual activity with a spouse, marriage forbids previously allowed sexual activity with others. This is a very significant change in how marriage works. (I shouldn't say that marriage in the past caused no new prohibitions. For instance, it implied a prohibition against flirting behavior with others than the spouse, etc. But in terms of sexual relations, marriage extended the options.)
I suspect that few advocates of same-sex marriage believe non-marital sex is always wrong. Therefore, they are already operating with a different concept of marriage from the traditional one. Moreover, if one sees marriage sexually as primarily constricting of permissions (prohibiting sex with persons other than the spouse), then it is no surprise that one will have less difficulty with the idea of same-sex marriage—after all, what is wrong, one may ask, with a couple voluntarily constricting what is permissible to them? Expanding what is permissible is a more serious issue.
Thus, I think it is very important that those who believe in the correctness of the traditional Western understanding of marriage not overfocus on the same-sex marriage issue. The issue of fornication—and of divorce for that matter—is probably at least equally important.